dm-verity: move data hash mismatch handling into its own function

Move the code that handles mismatches of data block hashes into its own
function so that it doesn't clutter up verity_verify_io().

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>